Residential and small commercial sewage treatment systems are subject to ever tightening treatment standards. In response to these evolving stricter treatment standards, treatment systems are becoming more complex. Prior art treatment systems include septic systems and aerobic treatment systems. Generally, aerobic treatment systems include an aeration zone and a clarifier zone, as epitomized by U.S. Pat. No. 4,650,577 to Hansel. The Hansel device includes: (1) an aeration zone where air is injected into the wastewater, creating a turbulent volume where mixing occurs; and (2) a clarifier structure, creating a separate quiescent zone within the interior of the clarifier structure to allow for settlement of suspended solids. The Hansel aerobic treatment system is one example of a suspended growth treatment system, called such because the treating bacteria mass is suspended within the fluids.
Another type of aerobic treatment system is a fixed film treatment system. The fixed film systems generally employ a bioreactor containing growth media that is suspended in the wastewaters. The growth media are structures (such as sheets of fiberglass or plastic, or small floating spheres or cubes) through which wastewater can flow over or through and present a surface area to which the bacteria can attach and grow on. Examples of such fixed film reactors are shown in U.S. Pat. Nos. 5,484,524 to MacLaren and 5,308,479 to Iwai. In these types of systems, the bioreactor's film is fixed. These types of systems are also considered aerobic treatment systems as air is injected into the wastewater system to allow for aerobic digestion by the bacterial mass.
Later developments in aerobic treatment systems include combining the aerobic treatment chamber with additional staged treatment tanks. For instance, U.S. Pat. No. 6,200,472 to Johnson discloses a central aerobic treatment tank combined with two additional tanks: (1) a pretreatment tank inserted into the system before the aerobic treatment tank, and (2) a post treatment tank inserted into the system after the aerobic treatment tank. The pretreatment tank operates as a settling tank or trash tank. Additionally, because air injection does not occur in the pretreatment tank, the wasters in the pretreatment tank are contained in an anaerobic environment whereby nitrate reduction is favored. The post treatment tank functions as an extended “external” clarifier, adding storage capacity for treated waters. The post treatment tank can also operate as a dosing tank that allows for the controlled release or discharge of effluent, when waters in the dosing tank reach a predetermined level. Such a system is described in Johnson, U.S. Pat. No. 6,200,472.
There are a number of “three tank” configurations in the industry. The general design is that of three tanks or zones arranged in a linear, sequential layout: a first pretreatment tank, joined to a second aerobic treatment tank; the second aerobic tank joined to a third post treatment or dosing tank. These treatment systems are designed to be buried at the installation site. Much effort has gone into joining three tanks into a single unitary linear unit. “Unitary” is used in the sense of a single structure which may be composed of a series of joined structures (such as multiple tanks) integrated into a unit which can be transported as a single unit. For instance, two tanks physically separated but fluidly connected by a pipe are not considered a unitary structure.
The manufacturer builds a unitary three tank system at the manufacturing plant, and delivers it pre-plumbed to the distributor or installer. Because the internal plumbing (that between the three tanks) is assembled by the manufacturer, each system can be tested for leakage prior to shipping thereby reducing the risks and costs of customer callbacks. This is an important feature, as installation includes burying the system on site, and later discovered leaks are expensive to repair. The installer need only to connect the influent and effluent plumbing in to and out of the system at the installation site, and test the same for leakage.
An added benefit of a single unitary tank is that this system presents a smaller footprint, reducing the size of the burial hole than would be required for multiple tanks joined on site. Exemplary three tank or three zone linear systems are shown in U.S. Pat. Nos. 6,406,619 to Johnson; 6,200,472 to Johnson; 6,180,004 to Drewery; 5,055,186 to Van Toever; and 3,907,672 to Milne, all incorporated by reference.
However, because the three tanks are generally laid out in a linear fashion, joined edge to edge (such as shown in the Johnson or Drewery patents), the footprint of such a linear arrangement is an elongated oval shape or rectangular shape. This elongated shape makes it difficult for the manufacturer to ship multiple units on a flatbed. While the unitary three tank system elongated footprint is smaller than that of three individual tanks joined at the installation site, it would ease transportation costs if the footprint could be further reduced as the elongated unit makes it difficult for the manufacturer to ship multiple units on a flatbed.
To further reduce the footprint and also to reduce the weight of unitary three tank systems, some manufacturers construct the three tank system with shared common walls between the tanks. Such a shared wall design is shown in the Johnson patents. However, the connection or join along the common walls (between the end tanks and center aerobic tanks) creates an increased possibility of leakage at these joins.